1. Field of the Invention
The present invention relates to a vibration detection device and a camera having a vibration detection function in a still camera, a video camera, and the like.
2. Related Background Art
Conventionally, a camera having a vibration detection function, and particularly, a camera having a vibration reduction function have been proposed in which a camera shake generated in the camera is detected by a vibration detection circuit using an angular velocity sensor or the like, and the optical axis of the photographing optical system is changed in accordance with the detection amount, thereby suppressing the image blur. As a means for changing the optical axis of the photographing optical system, for example, a correction lens as one of the photographing lenses is shifted to change the optical axis. The means for changing the optical axis of the photographing optical system is driven by an actuator such as a motor. More specifically, when a motor is used, rotation of the motor is reduced by a gear or the like, and rotation of the gear is converted into a linear motion, thereby driving the correction lens.
One example of the vibration reduction operation will be described with reference to a circuit diagram shown in FIG. 22. FIG. 22 is a circuit diagram for schematically explaining the prior art in association with a portion related to the present invention. A vibration detection circuit 5 detects a vibration generated in a camera. The vibration detection circuit 5 detects an angular velocity generated in the camera by using, e.g., an angular velocity sensor, thereby outputting a signal proportional to the angular velocity. The output from the vibration detection circuit 5 is output to a CPU 1 constituted by a one-chip microcomputer. The CPU 1 controls a motor 4 by a motor drive circuit 2 in accordance with the output value from the vibration detection circuit 5. With this operation, a correction lens 8 in photographing lenses 6 to 9 is shifted to change the optical axis, thereby canceling the vibration on the image surface. A lens velocity detection circuit 3 for detecting the position and shift velocity of the correction lens 8 detects the actual shift velocity of the correction lens 8 at this time, and feedback control is performed by the CPU 1 in accordance with the detected velocity. As the shift directions of the correction lens 8, normally, two axes perpendicular to each other and also perpendicular to the optical axis are necessary to drive the correction lens 8 in a plane perpendicular to the optical axis. However, the mechanisms for shifting the lens in these directions have the same arrangement, and only the mechanism for one axis is illustrated in FIG. 22. When an angular velocity sensor is used, an output value from the vibration detection circuit 5 changes in accordance with the angular velocity generated by the vibration of the camera. The CPU 1 converts (A/D-converts) the output value from the vibration detection circuit 5 into a digital value by an A/D converter with an 8-bit resolution, which is incorporated in the CPU 1, thereby detecting the angular velocity generated due to the camera shake. Since the vibration must be detected in real time, an output from the vibration detection circuit 5 is A/D-converted at a relatively short predetermined sampling interval of, e.g., 1 ms. Rotation of the motor 4 is converted into a linear motion by an appropriate gear or the like (not shown) to drive the correction lens 8.
However, the vibration reduction system with this arrangement has the following problems. The resolution of the A/D converter incorporated in the one-chip microcomputer used in a camera having the conventional vibration reduction function is as low as 8 bits. The operation of the A/D converter with an 8-bit resolution, which is incorporated in the CPU 1, will be described below with reference to FIG. 23. In the A/D converter incorporated in the CPU 1, the output terminal of the vibration detection circuit 5 is connected to the input side (A/D input) of a signal to be A/D-converted, and a reference voltage for A/D conversion, which serves as a reference voltage for A/D conversion, is applied to the other input terminal from the outside of the CPU 1. The output voltage from the vibration detection circuit 5 is converted into a digital value of 8 bits (values 0 to 255 in decimal notation) using the reference voltage for A/D conversion as a reference value, and the conversion result is stored in an A/D conversion result register. The data stored in the A/D conversion result register can be read out via an internal data bus as needed by executing a program written in a ROM (Read Only Memory) in the CPU 1. The relationship between the voltage input to the A/D input terminal and the digital value obtained upon A/D conversion is shown in FIG. 3. When the A/D input voltage is 0 V, the A/D conversion result obtained upon conversion by the A/D converter is 0. When the same voltage as the reference voltage for A/D conversion is input, the A/D conversion result is 255. Digitized values between an input voltage of 0 V and the reference voltage for A/D conversion are 256 levels, so that an A/D conversion result corresponding to an A/D input voltage can be obtained.
An angular velocity due to a camera shake often falls within a range of -20.degree./sec to +20.degree./sec under the normal use conditions of a compact camera. The vibration detection circuit 5 is a circuit for detecting an angular velocity generated due to the camera shake. Assume that the vibration detection circuit 5 outputs a voltage of 0 V when an angular velocity is -20.degree./sec, a voltage corresponding to 1/2 the reference voltage for A/D conversion in a stationary state, and the reference voltage for A/D conversion when an angular velocity is +20.degree./sec. Also assume that A/D conversion can be performed within an angular velocity range of -20.degree./sec to +20.degree./sec due to the normal camera shake. In this case, the angular velocity with respect to the quantization unit (1 LSB) of a digitized value with an 8-bit resolution corresponds to about 0.156.degree./sec. In the other word, an angular velocity of 0.156.degree./sec or less cannot be recognized by the A/D converter with 8-bit resolution. That is, this "unrecognizability" causes an error of 0.156.degree. in one second. Therefore, when a photographing operation is performed using a photographing lens having a focal length of 105 mm at a shutter speed of 1/4 second, an error of 105 mm.times.tan(0.156.degree.).times.(1/4 second).congruent.71 .mu.m is generated on the image surface. If the focal length of the photographing lens is large, or if the shutter speed is lower, a larger error is generated. Actually, since the output varies depending on the vibration detection circuit 5, vibration detection circuits do not output a predetermined output value even at a predetermined angular velocity. Additionally, the vibration detection circuit 5 often has an arrangement in which an output from the angular velocity sensor is amplified by an operational amplifier or the like and output. In this case, the lower limit of the output is only about 1 V, and the upper limit almost corresponds to (power supply voltage -1 V) because of the characteristics of the operational amplifier. For this reason, the vibration detection circuit 5 is designed to ensure a large margin with respect to a detection range of the angular velocity due to the camera shake. For example, assume that the power supply voltage of the vibration detection circuit 5 is 5 V, the reference voltage for A/D conversion is 5 V, an output range of the vibration detection circuit 5 is 1 to 4 V, and an output voltage of 2.5 V can be obtained in a stationary state. In this case, if an angular velocity of -20.degree./sec to +20.degree./sec, which is generated due to a camera shake, is to be detected, an angular velocity with respect to 1 LBS of the A/D converter corresponds to about 0.26.degree./sec. When a photographing operation is performed using a photographing lens having a focal length of 105 mm at a shutter speed of 1/4 second, an error on the image surface almost reaches 119 .mu.m. Normally, a vibration amount on the image surface must be about 50 .mu.m to obtain a picture with an ignorable image blur by a camera using a silver halide film. This error amount is very large.
There is a detection system for vibration reduction, in which an A/D converter with a 10-bit resolution is arranged outside a microcomputer in the vibration reduction system of a video camera so as to decrease the above-described error due to the resolution of the A/D converter. However, this system poses a problem of cost because of the externally arranged A/D converter. In addition, as compared to a one-chip microcomputer incorporating an A/D converter, a program for controlling the external A/D converter, e.g., a program for starting or stopping A/D conversion, or transferring an A/D conversion result to the one-chip microcomputer through communication with the A/D converter is additionally required. Furthermore, to perform vibration reduction, an output from the vibration detection circuit 5 must be detected in real time, and the correction lens 8 must be controlled in real time in accordance with the detected vibration amount, as described above. If a shutter (not shown) and the like are to be controlled by a single one-chip microcomputer, this control process is also required. Control of the external A/D converter increases processes. A high-performance one-chip microcomputer capable of simultaneously performing these control processes is very expensive. In A/D conversion of an output from the vibration detection circuit 5, some A/D converters require a long time for A/D conversion until the result is stored in an A/D conversion result register. Some A/D converters require a time of about 100 .mu.sec at maximum. The CPU 1 performs a process of detecting a vibration generated in the camera from the output from the vibration detection circuit 5, and at the same time, performs a vibration reduction process in which the correction lens 8 is shifted to correct the camera shake. When these processes are performed after the end of A/D conversion, the process may be delayed.
Additionally, an output from the vibration detection circuit 5 is a vibration signal (normally, at a frequency of 1 to 15 Hz) generated in the camera, which vibration signal is overlapped with high-frequency noise generated in the angular velocity sensor constituting the vibration detection circuit 5, the amplifier for amplifying the signal from the angular velocity sensor, and the like. Because of this high-frequency noise, even an A/D converter with a high resolution cannot exhibit its best performance.